The invention relates generally to an improvement in gauge control in a rolling mill using eccentricity estimates as disclosed in U.S. Pat. No. 4,222,254 to King et al. The disclosure of the patent is incorporated herein by reference.
It has been found that the process described in controlling the gauge of material or product exiting a rolling mill disclosed in the above-mentioned patent is somewhat unstable at travel speeds of the material in excess of about 2,000 feet per minute (fpm). The cause of the problem lies in the inherent lag in the response of transducers or cells in the mill stands that measure the forces or loads at which the material in the mill is rolled, and the inherent lag in the response of actuator mechanisms, such as hydraulic cylinders in the mill stands that mechanically control the loads. At relatively low operating speeds, the frequencies of the eccentricity disturbance are limited to a range of values that allows the force transducers or load cells to provide a measurement signal substantially in phase with the actual force variations occurring in the mill due to this disturbance. Additionally, this range of frequencies is also within a band in which the hydraulic cylinders can respond in phase with control signals in the process of moving the roll assembly of the mill in a manner that offsets the effect of the eccentricity. At higher operating speeds, however, the cells and cylinders cannot respond as fast as the occurrence of the eccentricity disturbance and command signals such that the delay in their response is incorporated in the signals that control the actuator mechanisms (cylinders) of the mill. When this delay becomes too large, the system becomes unstable creating undesired variations in the material leaving the mill. The delay in response is a phase delay of a certain number of degrees on a sign curve representing the eccentricity disturbance, the exact amount of the delay (degrees) depending on the rotational speed of the rolls and the dynamic characteristics of the response of the load cell and actuator mechanisms. The dynamic responses of the load cells and actuators involve both an initial "dead time", in which the load cells and cylinders do not respond at all, and the actual length of time of response of the cells and cylinders after they begin to respond. For example, in one test that led to the subject invention, a 12 millisecond delay (dead time) in the reaction of the mill's hydraulic cylinders, at a backup roll frequency of 3.4 Hz, resulted in a 14.7.degree. phase delay in the control signal generated in the system disclosed in the above-mentioned patent. The total delay in degrees was 65.8.